The present invention relates to an optical module, an optical element unit for the optical module and a method for determining the position and/or the orientation of an optical element unit. The invention can be used in connection with microlithography employed in the fabrication of microelectronic circuits.
In particular in the area of microlithography, in addition to the use of components made with the highest possible accuracy, it is necessary, among other things, to position the components of the imaging device, for example the optical elements such as lenses or mirrors, as precisely as possible, in order to achieve a correspondingly high image quality. The high requirements on accuracy, which are in the microscopic range of the order of magnitude of a few nanometers or less, are not least a consequence of the constant need to increase the resolution of the optical systems used in the fabrication of microelectronic circuits, in order to drive forward the miniaturisation of the microelectronic circuits that are to be produced.
With the increased resolution and, therefore, as a rule the concomitant decrease in wavelength of the light used, it is not only the requirements on the positional accuracy of the optical elements used that increase. Naturally, there is also an increase in the requirements with respect to minimising the imaging errors of the whole optical arrangement.
To minimise the imaging errors, it is known, for example from U.S. Pat. No. 6,842,277 B2 (Watson; the complete disclosure of which is incorporated herein by reference) and U.S. Pat. No. 7,443,619 B2 (Sakino et al.; the complete disclosure of which is incorporated herein by reference), to actively deform the optically active surfaces of one or a plurality of optical elements of the imaging system, in order to correct wavefront aberrations. To this end, for example, a number of actuators supported by a support structure of the mirror engage a back side of a mirror and introduce the corresponding forces and/or moments into the mirror, in order to achieve a desired deformation of the mirror surface relative to a setpoint geometry or rigid body geometry of the mirror.
These forces and/or moments for deformation of the mirror surface may as a rule also lead to a change in the position and/or orientation of the mirror (insofar as it is regarded as an infinitely rigid body), which in its turn must be corrected. The terms rigid body position or rigid body orientation of the optical element are often used in this context.
Correction of the position and/or orientation of the optical element is typically based on the measuring signals of one or a plurality of sensors, which detect the position and/or orientation of the optical element with respect to at least one reference, as is also known from U.S. Pat. No. 6,842,277 B2. A critical factor is that the deformation can cause a relative movement in the region of the measurement point, although the rigid body position or rigid body orientation of the optical element has not changed, or not to the extent captured by the sensors, so that corrections of the position and/or orientation of the optical element are performed, although these are not required.
A disadvantage of the solution proposed in U.S. Pat. No. 7,443,619 B2 is that the optical element and the deformation device form one unit, which in its turn has to be corrected in its position and/or orientation. If this correction of the rigid body position or orientation must take place very quickly, a heavy unit is a disadvantage. Moreover, rapid correction is hampered by cables or the like, which supply the deformation device. Moreover, such a unit requires considerable space, so that attainment of a high resonant frequency of the unit, which would be necessary for favourable control behaviour, becomes much more complicated.